Sound barrier

Call me crazy, but my fiance has been watching a lot of UFO stuff lately. I am extremely skeptical, but it put a question into my head, that I can not seem to find an answer to in my random searches on google.

Can an object that makes no sound while accelerating create a sonic boom when breaking the sound barrier?

Whenever I read about the sound barrier I find lots of answers on physics theory, calculations, examples, pictures, etc. All things that I have found refer to aircraft breaking the speed of sound. Well we all know how loud planes are, and I can understand the loud sound when a sound is generated by the aircraft itself.

But what about, theoretically, a propulsion system that is completely silent? Any help?

You should note that the creation of a sonic boom has nothing what-so-ever to do with propulsion system of an object. A sonic boom is a result of was is termed a shock. This shock is formed from the pressure waves created by the object simply moving through the air.